Sass negative variable value?

Question

I have a couple of scss selectors where I use the same amount positive and negative, as in:

padding: 0 15px 15px;
margin: 0 -15px 20px -15px;

Answer 1

I'm adding my two-penneth after considering the two previous answers, but then reading this (emphasis mine):

You should especially avoid using interpolation like #{$number}px. This doesn’t actually create a number! It creates an unquoted string that looks like a number, but won’t work with any number operations or functions. Try to make your math unit-clean so that $number already has the unit px, or write $number * 1px.

Source

Therefore I believe the correct way would be as follows, which preserves the mathematical abilities of SASS/SCSS:

$pad: 15px;
padding: 0 $pad $pad;
margin: 0 $pad*-1 20px $pad*-1;